A license plate reader that reads a license plate (vehicle number) of a vehicle running on a toll collection lane (road) of a tollgate is used in a toll collection system referred to as an ETC (registered trademark) system for automatically collecting a toll nonstop with respect to a vehicle running on a toll road such as an expressway.
The license plate reader takes an image of a license plate (vehicle number) from a front or a rear of a running vehicle, and recognizes the vehicle number on the license plate by processing the obtained image. A vehicle number image pickup device that takes the image of the vehicle number is used.
Recently, referring to the vehicle number image pickup device, a high-resolution camera of 2,000 pixels or more in a horizontal direction has come into the market as an image pickup means for taking the image of the vehicle number (license plate) such that a resolution sufficient to read the vehicle number can be obtained even when an image of a wide field of 1 lane or more (1 lane is about 3.5 m) is taken.
In addition, even though a digitized image is output from the high-resolution camera, a camera of a high dynamic range having a digital output of 12 bits or more has also come into the market, and there is a possibility that complex exposure control is unnecessary. However, in terms of versatility, processing time, and cost, it is not practical to use such a large-capacity image.
On the other hand, there is a publically known scheme in which a vehicle is detected from images continuously sent from a camera, only an image of a license plate portion is extracted from the detected vehicle image, and the extracted image is used as a vehicle number recognition image.
However, in such a technology, in general, an extremely short time is taken from when an approaching vehicle is detected until an image of a vehicle number is taken, and thus it is difficult to achieve high-performance image processing for a short period of time. As a result, there is a risk that an accurate position of a license plate may not be extracted.